The evolution in design of military aircraft is generally determined by the aircraft mission and the type of threat which the aircraft will encounter. For example, most military aircraft are dependent for propulsion on gas turbine engines which typically produce hot surfaces and a hot exhaust plume, emitting an infrared signal which makes the aircraft vulnerable to attack by heat seeking weapons. Consequently, various suppression systems have been proposed for reducing the infrared signal emanating from either the hot exhaust plume or the hot parts. Typically, the means for reducing the infrared signal has involved mixing cooling air with the engine exhaust to lower its temperature and thus the plume infrared signal, as well as incorporating sufficient blockage via baffles or turning in the exhaust stream to prevent a direct "line of sight" to the hot engine parts, as shown, for example, in U.S. Pat. No. 3,921,906 to Nye et al.; U.S. Pat. No. 3,981,448 to Demogenes et al.; and U.S. Pat. No. 4,198,817 to Fijita et al. A cooling duct is usually provided around the engine to insulate the engine and prevent the hot metal parts from being exposed at the surface of the aircraft.
Another threat which influences the design of military aircraft is the radar cross section (RCS) which, due to material type or shape, generates a radar return signal which indicates the presence of the aircraft and/or can be used for directing radar guided weapons to the aircraft. To minimize the radar cross section of the aircraft, surface structures and apertures are typically designed to minimize their radar reflection characteristics. In addition, radar absorbing coatings or materials of construction are used to reduce radar reflection.
However, one of the areas where difficulties occur in reducing the radar cross section is in the exhaust system. In general, reducing the radar cross section of any cavity is accomplished by reshaping and/or by applying special material coatings over the cavity surfaces. However, conventional radar absorbing materials cannot be used in the area around the infrared suppression system due to the resulting high temperatures associated with that treated area of the suppressor, which would damage the coatings or materials.
The shaping of the suppression system is normally determined by physical constraints required for optimizing engine efficiency and infrared suppression, such as gas flow, back pressure minimization, etc., which inherently results in a system susceptible to generating a strong radar return signal. Thus, the typical shaping methods for reducing radar cross section cannot be applied to conventional suppression systems as they would detrimentally effect engine efficiency or the systems' ability to effectively suppress the infrared signal. Consequently, a suppression system for a gas turbine engine which minimizes the threat from both heat seeking and radar guided weapons needs to be developed.